1. Field of the Invention
The present invention relates to a manufacturing method of a tire forming mold for vulcanization forming a tire and the tire forming mold.
2. Description of the Related Art
The tire forming mold mentioned above is constructed by combining a plurality of mold portions, and can be roughly classified into a two-piece type and a segmented type based on the mold structure. In the latter, an annular mold portion coming into contact with a tread surface of a tire is constructed by a plurality of sectors divided in a tire circumferential direction, and an inner peripheral surface of each of the sectors is provided with a frame portion for forming a groove portion and a concave portion for forming a land portion. Further, in the case where a notch called as a sipe is formed in the land portion, a thin blade is provided within the concave portion. The sectors are away from each other in a mold open state, however, the sectors get together so as to be connected as a circular ring shape in a mold clamping state and confront end surfaces with each other.
The sector is classified into an integral type and a piece type based on its structure. The integral type sector is integrally manufactured by an aluminum or the like as disclosed in Japanese Unexamined Patent Publication No. 56-58817. On the other hand, the piece type sector is constructed by incorporating a lot of pieces which are previously manufactured individually into an inner peripheral side of a back segment as disclosed Japanese Unexamined Patent Publication No. 2006-159669. In the piece type, it is possible to discharge an air from a lot of small gaps formed between the pieces, however, there is a tendency that the number of the parts is increased.
In the segmented type tire forming mold, since an end surface of the sector travels down through the frame portion forming a transverse groove or cuts across the blade in cooperation with a complexity of a tread pattern in the case of dividing the annular mold portion by a flat surface (dividing by a flat surface including a tire axis), a breakage of a thinned frame portion and a shortened blade comes into problem. On the contrary, in the case of employing a non-flat surface division, it is possible to appropriately avoid the thinning of the blade portion or the shortening of the blade by forming an end surface of the sector by a curved surface or a zigzag surface or the like, and it is possible to solve the problem mentioned above.
As mentioned above, in the light of suppressing the breakage of the frame portion and the blade, it is said that the non-flat surface division is more advantageous than the flat surface division. However, since the non-flat surface division has a high difficulty for working in comparison with the flat surface division, it is not easy to finish the end surface of the sector at a high precision. If the working precision of the end surface of the sector is low, a gap is formed at a time of confronting the end surfaces with each other, and a rubber burr is generated on the tread surface of the tire at a time of vulcanization molding. Accordingly, it becomes important to closely attach the end surfaces of the sectors to each other at a high precision even in the non-flat surface division.
Japanese Unexamined Patent Publication No. 56-58817 describes an apparatus for manufacturing an integral type sector by dividing in a non-flat surface manner an integrally cast molded ring-shaped casting. However, this apparatus is provided with a large-sized rotary table for retaining the ring-shaped casting, which is a heavy load, so as to move in a circumferential direction, and a large-sized NC machine, and thus requires a large-scaled expensive equipment.
On the other hand, in the piece type sector as described in Japanese Unexamined Patent Publication No. 2006-159669, since an individual piece has a small size, it is possible to work an end surface as a non-flat surface by using a general purpose working apparatus such as a wire electric discharge machine. However, since a working error is generated in each of the end surfaces in the case of working the end surfaces of a pair of pieces which are individually manufactured, a gap tends to be formed at a time of confronting them. Accordingly, it is necessary to carry out a lapping work after working the end surface as the non-flat surface, which raises a problem that the work is complicated.